Generally, sifters utilized within a dilute phase pneumatic conveying system, include, for instance, a pressurized container with a screen or screens of desired mesh size therein. Commonly, the container is mounted in such a manner that it is vibrated to cause the materials provided at the input of the container to move across the screen and thereby to effect the sifting action.
As shown and described by Krolopp, et al, U.S. Pat. No. 3,796,312, issued Mar. 12, 1974, there are essentially three courses of flowthrough in a typical screener; one for oversized material, a second for undersized or desired material and a third for air flow used to convey the undersized product. As shown by the Krolopp system, air and solid material traverse an inlet pipe at the top center of the container. As air enters into the container, since the container is much larger than the inlet pipe, such air will be reduced in velocity as it travels downward into the container. Nevertheless, the particulate materials moving with the air are still travelling at full velocity, and are thereby impinging the screen at high velocity sufficient to break up some of the unwanted contaminants (bugs, pieces of glass, pieces of trash, etc.) so that they pass through the screen mesh. As to the contaminants that do not pass through the screen, they are forced to the periphery of the screen in the same manner as oversized material.
Another feature shown by the Krolopp structure is the use of a by-pass screen for by-passing the converying air, which because of the design turns out to be very small in area, which allows it to possibly become plugged.